Hi Gil,

Here is the message in full. The acid comments are related to the thread it was in, not you !


All the slow guys at the back: Listen carefully.

If typical modulation frequencies are only say 49,4.9, 0.49 and .049 Mhz, these would yield wavelengths of 10, 100,1000 and 10,000 feet, by the time you have allowed for the double path the light travels.

It is easy to see that by increasing the higher frequency to the 500 Mhz level, wavelength drops to 1 feet. These are modulated onto the light source. Sub-millimetre precision is possible, but harder to do, thogh nothing like as hard a a TOF type measurment. Remember this is a continuous measurement.

Lets make a big instrument to measure long ranges. Lets say it reads 3427.14 feet. The last three digits are obtained when a phase meter is measuring with the highest frequency (49Mhz) 7.14 is equivalent to a phase shift of (7.14/10)*360 = 257 degrees. Then we send the next higher frequency (4.9Mhz) and measure the fractional phase yielding the nmsD or 2, then we use 0.49Mhz and measure to get 4, then 0.049Mhz, measure and get 3

Think a bit harder before you post assine replies. You can do it with only modest frequencies. Read some basic physics on wave properties. The key is PHASE measurement. You do not need to be even in the very high RF to do this. All you are doing is getting the fractional difference in phases measured at different frequencies. You can usually measure to 1 degree or slightly better without to much hassle.

This is a standard technique and can be done with a very cheap micro indeed if you know what you are doing. The key is in the rest of the electronics and particularly the optics.

Try a proper text book like Elementary Surveying, (Wolf and Brinker), 1989 pp88-95 for a nice simple explanation.

People get to bogged down in thinking in LIDAR terms.

Most total station instruments made before the advent of GPS use the technique, as does the new BosCh Laser rangefinder, which is accurate to 3mm in 20 metres.


Steve